Amateur porn filmed on US public train goes viral

US transit police are investigating an amateur porn video showing a man and woman having sex on a public train after it went viral online.

The footage showing the couple having sex on a seat inside a moving train in San Francisco, California, has been viewed more than 1.5 million times since it was posted on Facebook and other sites last week, Mercury News reports.

It is not clear whether other passengers were in the carriage while the illicit clip was being made.

Bay Area Rapid Transit Police said they are reviewing the video, which appears to have been shot on a mobile phone, and officers would be on the lookout for the amateur porn stars.

"Lewd activity is not only highly inappropriate on BART, it is illegal. It can and will get you banned from the entire system," said Bay Area Rapid Transit (BART) spokeswoman Luna Salaver.

Under a highly publicised new state law, the transit police have the authority to ban people from using the train network for up to a year if they are caught committing crimes.

Numerous people have been banned since the laws came into force on May 6 for infringements ranging from fighting, displaying weapons and selling drugs to attempted kidnapping.